EP0371329A2 - Titanium dioxide, process for its preparation and its use - Google Patents

Abstract

The present invention relates to titanium dioxide, to a process for its preparation, and to its use as an opacifier in glass-like enamels and melt glazes.

Description

The present invention relates to titanium dioxide, a process for its preparation and its use in glass-like enamels and glazes as opacifiers.

Titanium dioxide is produced as a colored pigment either by reacting TiCl₄ with O₂ or by hydrolysis of an aqueous solution of titanyl sulfate containing sulfuric acid and subsequent calcination.

In the case of the sulfate process, the titanium dioxide hydrate suspension is calcined in a rotary tube furnace with residence times of 5 to 20 hours. The precipitate resulting from the hydrolysis consists of fine particles with a specific BET surface area of 50-500 m² / g, which undergo strong agglomeration during the calcination.

The agglomerates resulting from calcination form particles of 0.1 to 1.0 mm, which are composed of primary particles of 0.2 to 0.4 µm. This material is not very suitable as a clinker or as a ground pigment for incorporation into glassy enamels or enamels, since it clumps when it is mixed with the other constituents of the enamel frit in a dry state, resulting in an incomplete and inhomogeneous solution of the titanium dioxide during the The frit melts.

In German Auslegeschrift 1 207 363 a process for the production of a TiO₂ product for incorporation in glass-like enamels by calcining precipitated titanium dioxide hydrate is described. A precipitated titanium dioxide hydrate with a coarse particle size, which consists of primary particles of 5 to 20 μm, is mixed with a precipitated titanium dioxide hydrate, which consists of primary particles of primarily less than 1 μm, as is produced in the production of TiO₂ pigments, and the mixture is calcined. The disadvantage of this method is that it is very expensive due to the two very different titanium dioxide hydrates that have to be prepared first. Furthermore, the disadvantage of TiO₂ obtained in this way is that it is characterized by a high bulk density of 1 to 2 kg / l and has a low specific surface area of well below 1 m² / g, which leads to low reactivity when melting in the frit.

In US Pat. No. 2,721,787, free-flowing, non-dusting aggregates consisting of calcined TiO 2 (primary particle size 0.2-0.3 μm, specific surface area 4.5-9 m² / g) and 0.1 to 3% of an inorganic binder are described . These aggregates are produced by spray-drying aqueous slurries of hydro-classified titanium dioxide pigments, which originate from normal pigment production, at 150 to 400 ° C. The disadvantage of this process is that calcined titanium dioxide pigments have to be prepared first, which are then taken up in water to produce a slurry, in order to then spray-dry this slurry in order to obtain the claimed aggregates.

German Patent Application 1 202 259 describes a process for the preparation of TiO₂ agglomerates for enamel frits by granulating suitable titanium dioxide (0.2-0.7 µm crystallite size) with the addition of a hydrolyzable metal or silicon halide.

In Japanese application 85/220884 spherical, fine TiO₂ particles are mixed with a TiO₂ sol, spray dried at 50 ° C and calcined at temperatures above 500 ° C. The TiO₂ sol is obtained from sulfuric acid-containing TiO₂ hydrate, which is neutralized with ammonia, filtered and washed, and with concentrated sweat rock acid is peptized. The product produced has grain sizes of 0.1 to 50 µm and is used in cosmetic products, catalysts and ceramics. It is not suitable for incorporation into emails. In the case of calcination below 800 ° C, even larger amounts of sulfate remain, which lead to rough, matt enamel surfaces when firing enamel. At higher calcining temperatures, the powder grains are too hard, which means that their digestion behavior in the frit is incomplete.

British Patent 1,176,912 describes a finely divided anatase with a sulfate content of 0.45-1.2%, based on TiO₂, a BET surface area of 35 to 55 m² / g and a primary particle size of 0.025-0.05 µm . The production is carried out by calcining a sulfuric acid-containing TiO₂ hydrate at 725 to 760 ° C and subsequent grinding. The product is used in photography as well as in copy papers. It is not suitable for enamel incorporation due to its high sulfur content, which leads to rough, matt surfaces, as well as its fine particle size, which causes lumps in the frit raw material mixture and thus incomplete dissolving and sludge formation when the frit melts.

In German Offenlegungsschrift 1 592 428, titanium dioxide hydrate or preferably small titanium dioxide particles are chemically treated in a liquid to promote agglomeration, the particles agglomerate into microspheres from 1 to 500 μm and the agglomerate microspheres dried and calcined. The chemical treatment consists of a reaction with a mineral acid, an organ. Acid or a base. The bulk density of the TiO₂ powder obtained, which are suitable for use as a catalyst support, is 0.8 to 1.0 kg / l. The products are not suitable for incorporation into enamel, since the individual granules are very densely packed and hard as a result of the chemical treatment in connection with the subsequent agglomeration, which leads to poor and incomplete melting in the enamel frit.

The task was therefore to provide a suitable titanium dioxide, which can be used as an opacifier for glassy enamels and enamels.

It has now been found that the titanium dioxide according to the invention meets the above requirements. This titanium dioxide has a TiO₂ content of ≧ 87% by weight, a sulfate content of less than 0.45% by weight, preferably less than 0.3% by weight, in each case based on TiO₂, an alkali oxide content of less than 0 , 5 wt .-%, preferably less than 0.2 wt .-%, based on TiO₂, a residual water content of less than 12 wt .-%, based on TiO₂, a BET surface area of 30 to 500 m² / g, a bulk weight of 0.2 to 0.8 kg / l, a primary particle size of 0.005 to 0.1 µm and an average aggregate grain size of 30 to 500 µm and is in amorphous form and / or as crystalline anatase.

The TiO₂ content of the granules is ≧ 87% by weight, but can still be increased by heat treatment. The sulfate content is less than 0.45 wt .-%, preferably less than 0.3 wt .-%, based on TiO₂. Higher sulfate levels have an unfavorable effect on TiO₂ recrystallization during enamel baking and lead to irregular, matt enamelling.

The total content of alkali oxides, in particular sodium and potassium oxide, is below 0.5% by weight, preferably below 0.2% by weight, based on TiO₂. Higher proportions result in losses in the enamel cloudiness. A residual water content up to 12 wt .-%, based on TiO₂, does not affect the enamel properties of the titanium dioxide.

The high BET surface area of 30 to 500 m² / g (determination of the BET surface area according to DIN 66 131, section October 6, 1973; 5-point method, measuring gas: nitrogen, adsorption at the temperature of the boiling nitrogen, assumption of the space requirement a nitrogen molecule of 0.162 nm², pretreatment: 1 hour heating in a nitrogen stream at 130 ° C) and the small primary particle size of 0.005 to 0.1 µm have a high reactivity and thus an excellent digestion behavior of the TiO₂ in the enamel melt.

The 30 to 500 µm large, spherical aggregates show perfect flow behavior, do not stick when mixed with the other frit components and have one good stability, ie they do not disintegrate into fine grains during the mixing process, which would lead to clumping and incomplete melting.

When mixed, the aggregates are distributed evenly in the frit and then melt completely and evenly.

In the quenched frit, the particles form titanium dioxide-rich neighborhoods that act as germ cells for titanium dioxide recrystallization in the early stages of enamel firing. In the further course of the enamel firing, almost exclusively anatase in a suitable crystallite size, isometric shape and even distribution is produced, whereby a high reflectivity with excellent opacity is obtained.

In a particular embodiment, the titanium dioxide preferably contains oxides, phosphates and / or silicates of the metals Mg, Ca, Sr, Ba, Sn, Sb, Bi, Y, Zr, Zn, V, Nb, W and / or Mo and / or oxides of the elements P and / or Si in an amount of up to 10% by weight, preferably up to 7% by weight, based on titanium dioxide. For example, an addition of P₂O₅ improves the TiO₂ recrystallization and thus the turbidity in the enamel, an addition of WO₃ improves the color in the enamel. Surprisingly, the original, spatial removal of the additives from the TiO₂ grain is of great importance for the subsequent TiO₂ turbidity, that is, the introduction of the same Adding additives in the same amount directly to the frit would have a much smaller effect.

The titanium dioxide can additionally contain binders such as e.g. Water glass, carboxymethyl celluloses and / or polyvinyl alcohols and / or plasticizers such as e.g. Contain polyphosphates and / or polyacrylates in an amount of 0.1 to 5 wt .-%, based on TiO₂.

The invention furthermore relates to a suitable process for the preparation of the titanium dioxide according to the invention, in which the precipitated, acidic titanium dioxide hydrate is brought to a pH of from 7 to 13 with a base, optionally heating for 0.1 to 10 hours to a temperature between 40 ° C. and reflux, is filtered and washed, the excess base is optionally removed, spray drying is carried out, if appropriate, with the addition of auxiliaries and, if appropriate, a temperature treatment at 200 to 800 ° C., preferably 400 to 600 ° C., follows.

The base treatment of the acidic TiO₂ hydrate has the goal of converting the adhering acid as completely as possible into the corresponding salt, which is then washed out. A prior 0.1 to 10 hour heating of the base-treated TiO₂ hydrate to a temperature between 40 ° C and reflux favors the salt removal.

In a special embodiment, the salt-containing TiO₂ hydrate can be subjected to a pressure treatment before washing. This can e.g. done by performing the temperature treatment in a closed pressure vessel. Also e.g. the high pressure relaxation technology can be used in suitable equipment.

The measures mentioned lead to an improvement in the filtration and foreign salt leaching, which results in an improvement in the enamel properties of the titanium dioxide treated in this way.

After filtration and washing, the excess base is preferably largely removed again. When using sodium hydroxide solution and / or potassium hydroxide solution, this is done by using an acid such as Hydrochloric, nitric and / or phosphoric acid is adjusted to a pH of 7 to 2 and the sodium and / or potassium salt formed is removed by renewed filtration and washing. Measures to improve filtration and washing, such as temperature or pressure treatment, are also possible in this step.

In a further preferred embodiment, ammonia is used as the base. The ammonium salt formed can also be removed after adjusting the pH to 7 to 2 using an acid and then filtering with washing. However, it can preferably also be removed by heat treatment after spray drying at 400 to 800 ° C., in which case the second filtration cycle is saved.

In a preferred embodiment, the washed, low-salt titanium dioxide hydrate can, in a preferred embodiment, be admixed in a suitable form before the spray drying, which in the end product predominantly as oxides, phosphates and / or silicates of the elements Mg, Ca, Sr, Ba, Sn, Sb, Bi, Y, Zi, Zn, V, Nb, W and / or Mo and / or as oxides of the elements P and / or Si in an amount of up to 10% by weight, preferably up to 7% by weight, based on titanium dioxide .

The auxiliary substances are salts of the above-mentioned metals such as carbonates, nitrates, chlorides, fluorides, silicates, phosphates or oxides.

Also binders such as Water glass, carboxymethyl celluloses or polyvinyl alcohols and / or plasticizers such as e.g. Polyphosphates or polyacrylates can be added to the TiO₂ hydrate in an amount of 0.1 to 5% by weight each before the calcination.

Spray drying takes place at an exhaust air temperature above 90 ° C, preferably above 140 ° C. For this purpose, the low-salt TiO₂ hydrate suspension is optionally mixed with auxiliaries and, with a solids content of Gew 25% by weight, preferably ≧ 35% by weight, optionally introduced into the drying tower via a single- or multi-component nozzle or a centrifugal atomizer disc. Both a high solids content of the feed suspension and a high drying temperature in the spray dryer have a favorable effect on the stability of the granules and thus on the processability of the TiO₂ by email.

Subsequent annealing of the spray-dried powder at 200 to 800 ° C, preferably 400 to 600 ° C, the TiO₂ content and the grain stability can be increased if necessary.

Precipitated, sulfuric acid-containing titanium dioxide hydrate with a BET surface area between 50 and 500 m 2 / g, as used e.g. arises in the pigment production process using the sulfate process.

The titanium dioxide according to the invention can be used as an opacifier in the production of enamels or glazes.

The invention is explained in more detail below with the aid of examples 1 and 2, without these constituting a restriction.

example 1

An aqueous titanium dioxide hydrate suspension with a TiO₂ content of 25.5% by weight, a sulfate content of 7.5% by weight, based on TiO₂, and a BET surface area of 280 m² / g is brought to pH 8 with sodium hydroxide solution, Heated to 90 ° C for 1 hour, filtered and washed free of salt, resuspended in H₂O, adjusted to pH 4 with hydrochloric acid, filtered again and washed free of salt. The filter cake with a solids content of 40% by weight is resuspended. 0.5% by weight of P₂O₅, based on TiO₂, are added to this suspension and then it is dried in a spray dryer with centrifugal disk atomization at an exhaust air temperature of 170 ° C.

The product obtained has a TiO₂ content of 92.0 wt .-%, a sulfate content of 0.2 wt .-%, based on TiO₂, an alkali oxide content of 0.1 wt .-%, based on TiO₂, a chloride content of 0.4 wt .-%, based on TiO₂, a P₂O₅ content of 0.5 wt .-%, based on TiO₂, an H₂O content of 6.7 wt .-%, based on TiO₂, a BET surface area of 280 m² / g, a primary particle size of 0.01 µm, a bulk density of 0.45 kg / l and an average aggregate grain size of 80 µm and is present as an amorphous and crystalline anatase.

The product does not stick, is free-flowing and shows excellent mixing and digestion behavior in a bortitanium white enamel frit. After order and branding maintain high reflectivity, high opacity and a favorable shade of the enamel.

Example 2

A titanium dioxide hydrate suspension as described in Example 1 is brought to pH 8 with ammonia, heated to 60 ° C. for one hour, filtered and washed free of salt. The residue is taken up in water to form a 35% by weight suspension. The mixture is then dried in a spray dryer with atomization at an exhaust air temperature of 140 ° C. and post-annealed at 500 ° C. for 30 minutes. The product obtained has a TiO₂ content of 97% by weight, a sulfate content of 0.25% by weight, based on TiO₂, an H₂O content of 1.7% by weight based on TiO₂, a BET Surface of 115 m² / g, a bulk density of 0.6 kg / l, a primary particle size of 0.03 µm and an average aggregate grain size of 185 µm and is predominantly in the form of crystalline anatase.

The product does not stick, is free-flowing and shows excellent mixing and digestion behavior in a bortitanium white enamel frit. After application and penetration, a high reflectivity, a high covering power and a favorable color of the enamelling are obtained.

Claims (7)

1. titanium dioxide, characterized in that it has a content of ≧ 87 wt .-% titanium dioxide, a sulfate content of ≦ 0.45 wt .-%, preferably <0.3 wt .-%, based on TiO₂, an alkali oxide Content of <0.5 wt .-%, preferably <0.2 wt .-%, based on TiO₂, an H₂O content of <12 wt .-%, based on TiO₂, a BET surface area of 30 to 500 m² / g, a bulk density of 0.2 to 0.8 kg / l, a primary particle size of 0.005 to 0.1 microns and an average aggregate grain size of 30-500 microns and is in amorphous form and / or as crystalline anatase. 2. Titanium dioxide according to claim 1, characterized in that it is predominantly oxides, phosphates and / or silicates of the metals Mg, Ca, Sr, Ba, Sn, Sb, Bi, Y, Zr, Zn, V, W and / or Mo and / or oxides of the elements P and / or Si in an amount of up to 10 wt .-%, based on TiO₂, contains. 3. A process for the preparation of titanium dioxide according to one of claims 1 or 2, characterized in that acidic titanium dioxide hydrate is adjusted to pH 7-13 with a base, optionally 0.1 to 10 hours of heating to a temperature between 40 ° C and reflux , filtered and washed, the excess base is optionally removed, spray drying, optionally with the addition of auxiliaries, he follows and optionally a temperature treatment at 200 to 800 ° C, preferably 400 to 600 ° C follows. 4. The method according to claim 3, characterized in that sulfuric acid-containing TiO₂ hydrate that is obtained after hydrolysis in the pigment production process by the sulfate process with a BET surface area between 50 and 500 m² / g. 5. The method according to claim 3, characterized in that the base used is sodium hydroxide solution and / or potassium hydroxide solution and the excess base is removed by adjusting to a pH of 2-7 with an acid and the resulting sodium and / or potassium salt filtering and washing again. 6. The method according to claim 3, characterized in that ammonia is used as the base and the excess ammonia and the ammonium salt formed are removed by temperature treatment at 400 to 800 ° C. 7. Use of the titanium dioxide according to claim 1 and 2 as an opacifier for glassy enamels and glazes.